This invention is in the field of hydraulic boosters; more particularly, the invention relates to the travel ratio feature of the hydraulic booster.
It is known in the art that hydraulic boosters contain a travel ratio feature by which the movement of an input means is translated to even greater movement of an output means. In hydraulic boosters for use with motor vehicle power braking systems, the movement of the input rod in response to the operator stepping on the brake pedal is translated into an even greater movement of the output rod. This translation is measured by the travel ratio, that is, the ratio of movement of the output rod to the movement of the input rod. Typical systems with hydraulic booster pedal travel controls are shown in U.S. Pat. Nos. 3,793,829 and 3,899,889.
The mechanical advantage as measured by a travel ratio can be attained in a hydraulic booster by use of a ratio chamber between a slave piston and a power piston which is connected to the output means.
When the input means is activated, the slave piston and the power piston move toward the output. The relative sliding movement between the power piston and the slave piston results from a change in the geometry of the ratio chamber. The ratio chamber containing an incompressible fluid is sealed at a constant volume. The geometry of the ratio chamber changes and results in greater movement of the slave piston relative to the movement of the power piston.
When there is no pressure for power assist by the hydraulic booster and the system is in manual mode, the power piston and slave piston move together and the ratio chamber decreases in volume. Fluid leaves the ratio chamber through a seal means which allows fluid to leave but not enter the ratio chamber. When the manual mode is released, the power piston and slave piston tend to return to the neutral position and the ratio chamber volume increases. The increase in volume results in a pressure drop. In prior art systems, this lower pressure exists until the ratio chamber reaches a point where it communicates with a supply opening so that fluid can be supplied to the ratio chamber and return it to proper pressure.
In the prior art system if the brake is not returned to the neutral mode and power assist braking mode becomes effective, the smaller volume of the ratio chamber results in a lower travel ratio. Further, even if the booster is in power assist mode, there is an initial operation before the ratio chamber and the supply opening communication is broken, when the advantage of the travel ratio is not effective since the ratio chamber loses volume to the supply opening. It is desirable to improve hydraulic boosters having the travel ratio feature to overcome these difficulties of the prior art systems.